White matter (WM) lesions, characterized by the loss of myelin and myelin-producing oligodendrocytes (OLs), are a major cause of functional disability after stroke but have not been widely appreciated in therapeutic studies until recently. Here we propose to rectify this gap in the field by focusing on WM integrity and its modulation by immune responses in the ischemic brain. Activated microglia/macrophages of distinct phenotypes are known to determine OL cell fate and WM integrity after brain injuries. Specifically, the alternatively activated M2 phenotype is essential for WM preservation and repair because M2 cells resolve local inflammation, clear broken myelin sheaths, and provide trophic factors that promote WM repair. CD4+CD25+ regulatory T cells (Tregs) are a specialized subpopulation of T cells that negatively regulate immune responses. Our recent study demonstrated that adoptive Treg therapy exerted early neuroprotection by targeting inflammatory dysregulation and neurovascular disruption after stroke. However, it is not known whether Tregs also have a beneficial effect on WM integrity. Recently, we discovered that Treg-conditioned media stimulates microglial polarization toward the M2 phenotype, and M2 microglia enhance OL survival and promote OPC differentiation in vitro. These exciting results suggest that Tregs can preserve WM integrity. We obtained further promising data showing that 1) Treg transfer at 2h of reperfusion reduced the extent of WM injury and improved sensorimotor functions for at least 28d after transient middle cerebral artery occlusion (tMCAO); 2) Post-stroke Treg treatment resulted in a long-lasting elevation of IL-10, a major Treg-derived cytokine that is important for WM repair; 3) Treg treatment promoted M2 polarization of microglia/macrophages in both WM and gray matter after tMCAO. Furthermore, we have successfully induced a robust increase of Tregs in the circulation after stroke by systemic injection of interleukin (IL)-2/IL-2 antibody complex (IL-2/IL- 2Ab), an established approach to expand Tregs in vivo. We demonstrated that IL-2/IL-2Ab-induced Treg expansion reduces myelin loss 7d after tMCAO and improves sensorimotor functions. The current proposal will further explore the effects of Tregs on WM injury and repair after stroke and develop in vivo Treg expansion as a novel strategy to promote WM integrity and enhance post-stroke recovery. The central hypothesis to be tested is that Tregs promote WM integrity and long-term recovery after stroke by polarizing microglia/macrophages toward the M2 phenotype in an IL-10 dependent manner. Three specific aims are proposed: Aim 1. Test the hypothesis that Treg treatment after stroke improves long-term functional recovery and promotes WM integrity. Aim 2. Test the hypothesis that Treg-derived IL-10 shifts microglia/macrophage polarization towards the M2 phenotype, thereby promoting WM integrity after stroke. Aim 3. Test the hypothesis that in vivo expansion of Tregs with post-stroke IL-2/IL-2Ab treatment is effective in reducing long-term WM injury and improving neurological recovery after stroke.